1. Field of the Invention
The present invention relates to both a ultra high precision length measuring machine utilizing laser beams which has a constitution such that graduations are marked on a physical medium in a longitudinal direction and controls the influence of a change with the passage of time or a working environment and a length measuring method.
2. Description of the Related Art
Heretofore, there have been two types of measuring instruments: one uses a length measuring machine; the other uses laser beams. In the case of length measurement using the length measuring machine, it is possible to perform stable measurement even while the length measuring machine is travelling in the air, but it has the following disadvantages.
(1) A physical medium is influenced by environmental temperature and therefore a part on which graduations are marked changes in length. For example, in the case of a steel medium, a precision decreases by 11.8 .mu.m/m.degree. C.
(2) In the case of a physical medium, the length of the part on which the graduations are marked becomes uneven when the medium is in a vertical position even though external force (gravitation) is stable. Thus, the precision decreases.
(3) The physical medium has a tendency to expand and contract with the passage of time. Thus, even though heat resistance glass which is insensitive to variations in temperature, for example, ZERO DURE MATERIAL(brand name) is used, the length changes in a longitudinal direction by 30 to 70 nm/m per year with the passage of time. This is equivalent to about 0.3 to 0.7 .mu.m/m in ten years which is a significant value.
On the other hand, in the case of a gage interferometer using laser beams, a physical property of laser beams to stabilize a wave length is demonstrated as it is in a vacuum, thereby enabling highly accurate length measurement. However, in the air, the gage interferometer is easily influenced by a fluctuation of air (fluctuation of refractive index). Thus, there is a limit in controlling an environmental influence, especially when the measurement is performed while travelling. Measuring instruments are usually used in a space in which air exists. It is reported that under such circumstances, slight movement of 2 .mu.m in an optical path length of about 1 meter leads to a fluctuation of measured data of about 2 .mu.m due to the influence of a fluctuation of air resulting from travelling of a material body. Thus, although a system which always makes corrections of the fluctuation of air extending over a whole optical path using laser beams of two wave lengths having very different frequencies has been put to practical use, the fluctuation of measured data can be controlled up to at most about .+-.0.01 .mu.m when the material body is at a standstill.
As described above, measurement using a length measuring machine or laser beams has a disadvantage, and therefore it is very difficult to perform stable and high resolution measurement of about 0.1 nm or 1 nm in a length measuring interval in the order of 1 meter, especially measurement of the location of a travelling object under ordinary circumstances (under the circumstances in which air exists).